This can be caused by several issues:
Are you are generating from a record of the main object of the form?
You can only generate output from a record of the main object of the form.
Is the Form filer filtering out forms?
Are you displaying all the forms or are the forms filtered?
Are you trying to generate with Generator permissions but there are no published Forms?
A user with Generator permissions can only see published forms (not activated forms).
Is Magic Mover for Notes &Attachments installed in your organization and does the conversion list include Docomotion objects?
When using Magic Mover for Notes and Attachments you need to remove Docomotion objects (API start with doco__). Using the tool on Docomotion objects result in errors or damage to the forms.